Roles of Phthalic Acid and Oleic Acid on Chemical Mechanical Polishing in Alkaline Slurries for Cobalt Interconnects

Cobalt (Co) with its low resistivity, excellent resistance to electro-migration with co-deposition property and void-free seamless fill ability, pledges to transform the landscape of integrated circuits in many areas, especially in logic contacts and interconnects. Chemical mechanical polishing (CMP), which utilizes the synergistic action of chemical corrosion and mechanical wear, has been recognized as the exclusive manufacturing approach to achieve global and local planarization of wafer surfaces. In this work, we proposed an innovative hydrogen peroxide (HP) polishing slurry, applying phthalic acid as the complexing agent and oleic acid as the corrosion inhibitor at pH 10. From the results of appropriate material removal/static etching rates as well as the smooth surface morphologies, phthalic acid and oleic acid are identified to be qualified in HP-based slurries for heterogeneous materials of Co interconnects. Moreover, the mechanisms of complexation and inhibition have been revealed by electrochemical analysis and X-ray photoelectron spectroscopy tests. It can be concluded that with the addition of complexing agent phthalic acid in HP-based slurries, the transformation of oxidization from low-valence compounds to high-valence was partially replaced by stronger complexing action, making the removal mechanism be attributed to the combined oxidization and complexation. When oleic acid is introduced, it could react with Co oxides to form a polymer protection layer adsorbing on Co films, achieving an inhibition effect to avoid Co excessive corrosion. Meanwhile, the oxidization of HP as well as the complexation of phthalic acid, combined with the protection of oleic acid play critical roles to maintain the appropriate removal rates and selection ratios.